1. Field of the Invention
This invention relates generally to spectroscopy and, more particularly, to a Raman probe having a small diameter immersion tip.
2. Description of the Related Art
Molecular spectroscopy is a family on analytical techniques that provide information about molecular structure by studying the interaction of electromagnetic radiation with the materials of interest. In most of these techniques, the information is generally obtained by studying the absorption of radiation as a function of optical frequency. Raman spectroscopy is unique in that it analyzes the radiation that is emitted (or scattered) when the sample is irradiated by an intense optical signal consisting of a single frequency, or a narrow range of frequencies. In this case the “Raman scattering” signal is essentially an emission spectrum with frequency dependent intensities. The individual bands in this spectrum are shifted from the frequency of the excitation signal by amounts that are related to the structure of the molecules present in the sample.
Many different probe designs have been proposed for use in Raman spectroscopy. Some examples are given in I. R. Lewis & P. R. Griffiths, “Raman Spectroscopy with Fiber-Optic Sampling”, Applied Spectroscopy. Vol. 50, pg. 12A, 1996, FIGS. 3 through 11. These fall into two general categories. The first category includes probes that use separate optical fibers to transmit radiation to and from the sample. Such “internal fiber probes” can be made quite small in diameter. However, they are deficient in that their design generally does not allow the use of optical filtering between the sample and the fibers to filter out the spurious Raman signals produced in the fiber. The second category includes probes which do not use internal fibers but which employ optical means to superimpose the path of the laser excitation beam and the receiving path for transmission two and from the sample. Although these probes are often employ optical fibers for coupling to the laser source and the spectrometer, their design allows for the use of filtering between these fibers and the sample. They are often referred to as “externally filtered” or “fully filtered” probes. A specific purpose of my invention is thus the design and construction of a fully filtered probe which is suitable for insertion into small volume chemical reaction vessels. I have been told verbally that previous attempts to design small diameter, fully filtered probes have been unsuccessful. This may be due to the fact that most previous probe designs have superimposed the transmitted and received paths in such a way that they are both collimated and have approximately the same diameters at the point where they are combined. This turns out to be a poor choice of conditions for a small diameter probe.
Model RFP-480 Raman Probe introduced in the year 2000 by my company, Axiom Analytical, employs a unique design in which a collimated laser beam is injected into the center of the receiving beam area by means of a rhomboid prism (see FIG. 1). This approach provides ease of optical alignment by taking advantage of the fact that the rhomboid can be fabricated with its two reflecting surfaces highly parallel. However, in order to avoid blocking a significant portion of the received signal, the areas of both the rhomboid and the injected laser beam are made quite small. As will be seen below, the use of a small diameter laser excitation beam provides the first step toward the successful design of a probe with an extended-length small diameter immersion tip. However, in the standard RFP-480, both the transmitted and received beams are nominally collimated in the beam-combining plane and inner diameter of the lightguide in the immersion tip is necessarily set approximately equal to the diameter of the lens which focuses the Raman shifted radiation onto the receiving optical fiber. I will show below that different considerations apply when it is necessary for the probe to have a small diameter immersion tip that can be inserted a substantial distance into a chemical mixture.